Reverse the ordering of words in a string

Question

I have this string s1 = \"My name is X Y Z\" and I want to reverse the order of the words so that s1 = \"Z Y X is name My\".

I can do it u

Answer 1

What language? If PHP, you can explode on space, then pass the result to array_reverse.

If its not PHP, you'll have to do something slightly more complex like:

words = aString.split(" ");
for (i = 0; i < words.length; i++) {
    words[i] = words[words.length-i];
}

Answer 2

In Python...

ip = "My name is X Y Z"
words = ip.split()
words.reverse()
print ' '.join(words)

Anyway cookamunga provided good inline solution using python!

Answer 3

You cannot do the reversal without at least some extra data structure. I think the smallest structure would be a single character as a buffer while you swap letters. It can still be considered "in place", but it's not completely "extra data structure free".

Below is code implementing what Bill the Lizard describes:

string words = "this is a test";

// Reverse the entire string
for(int i = 0; i < strlen(words) / 2; ++i) {
  char temp = words[i];
  words[i] = words[strlen(words) - i];
  words[strlen(words) - i] = temp;
}

// Reverse each word
for(int i = 0; i < strlen(words); ++i) {
  int wordstart = -1;
  int wordend = -1;
  if(words[i] != ' ') {
    wordstart = i;
    for(int j = wordstart; j < strlen(words); ++j) {
      if(words[j] == ' ') {
        wordend = j - 1;
        break;
      }
    }
    if(wordend == -1)
      wordend = strlen(words);
    for(int j = wordstart ; j <= (wordend + wordstart) / 2 ; ++j) {
      char temp = words[j];
      words[j] = words[wordend - (j - wordstart)];
      words[wordend - (j - wordstart)] = temp;
    }
    i = wordend;
  }
}

Answer 4

Better version
Check my blog http://bamaracoulibaly.blogspot.co.uk/2012/04/19-reverse-order-of-words-in-text.html

public string reverseTheWords(string description)
{
    if(!(string.IsNullOrEmpty(description)) && (description.IndexOf(" ") > 1))
    {
        string[] words= description.Split(' ');
        Array.Reverse(words);
        foreach (string word in words)
        {
            string phrase = string.Join(" ", words);
            Console.WriteLine(phrase);
        }
        return phrase;
    }
    return description;
}

Answer 5

This is not perfect but it works for me right now. I don't know if it has O(n) running time btw (still studying it ^^) but it uses one additional array to fulfill the task.

It is probably not the best answer to your problem because i use a dest string to save the reversed version instead of replacing each words in the source string. The problem is that i use a local stack variable named buf to copy all the words in and i can not copy but into the source string as this would lead to a crash if the source string is const char * type.

But it was my first attempt to write s.th. like this :) Ok enough blablub. here is code:

#include <iostream>
using namespace std;

void reverse(char *des, char * const s);
int main (int argc, const char * argv[])
{    
    char* s = (char*)"reservered. rights All Saints. The 2011 (c) Copyright 11/10/11 on Pfundstein Markus by Created";
    char *x = (char*)"Dogfish! White-spotted Shark, Bullhead";

    printf("Before: |%s|\n", x);
    printf("Before: |%s|\n", s);

    char *d = (char*)malloc((strlen(s)+1)*sizeof(char));  
    char *i = (char*)malloc((strlen(x)+1)*sizeof(char));

    reverse(d,s);
    reverse(i,x);

    printf("After: |%s|\n", i);
    printf("After: |%s|\n", d);

    free (i);
    free (d);

    return 0;
}

void reverse(char *dest, char *const s) {
    // create a temporary pointer
    if (strlen(s)==0) return;
    unsigned long offset = strlen(s)+1;

    char *buf = (char*)malloc((offset)*sizeof(char));
    memset(buf, 0, offset);

    char *p;
    // iterate from end to begin and count how much words we have
    for (unsigned long i = offset; i != 0; i--) {
        p = s+i;
        // if we discover a whitespace we know that we have a whole word
        if (*p == ' ' || *p == '\0') {
            // we increment the counter
            if (*p != '\0') {
                // we write the word into the buffer
                ++p;
                int d = (int)(strlen(p)-strlen(buf));
                strncat(buf, p, d);
                strcat(buf, " ");
            }
        }
    }

    // copy the last word
    p -= 1;
    int d = (int)(strlen(p)-strlen(buf));
    strncat(buf, p, d);
    strcat(buf, "\0");

    // copy stuff to destination string
    for (int i = 0; i < offset; ++i) {
        *(dest+i)=*(buf+i);
    }

    free(buf);
}

Answer 6

Here is the Java Implementation:

public static String reverseAllWords(String given_string)
{
    if(given_string == null || given_string.isBlank())
        return given_string;

    char[] str = given_string.toCharArray();
    int start = 0;

    // Reverse the entire string
    reverseString(str, start, given_string.length() - 1);

    // Reverse the letters of each individual word
    for(int end = 0; end <= given_string.length(); end++)
    {
        if(end == given_string.length() || str[end] == ' ')
        {
            reverseString(str, start, end-1);
            start = end + 1;
        }
    }
    return new String(str);
}

// In-place reverse string method
public static void reverseString(char[] str, int start, int end)
{
    while(start < end)
    {
        char temp = str[start];
        str[start++] = str[end];
        str[end--] = temp;
    }
}